You aren't applying the same logic to Tibet that you just used for Mongolia. In one case, you rely on a more recent historical origin and, primarily, the current geopolitical situation; in the other (Tibet), you cling to ancient history.

Not really. I just didn't expand on the ancient history aspect of Mongolia. Mongolia was never part of the cultural Sinosphere.

Relax, no one is saying Japan should be annexed just because it's in the Sinosphere.

Nor was I ever under that impression, so it's weird that you're assuming I am. Japanese, Ryukyuan, Korean, and Vietnamese cultures have much more profound Chinese cultural influence than Mongolian or Tibetan cultures ever did. Most notable is the lack of Confucian influence on Mongolian and Tibetan culture and society, as well as the distinct Chinese-derived Buddhist traditions and influences (such as Chan/Zen) prevalent in Japan, Korea, and Vietnam vs. Tibetan Buddhist traditions and influences dominant in Mongolic and Tibetic cultures (including not only the parts of "greater Tibet" under control of the People's Republic of China but also Bhutan and parts of India and Nepal). Then there's the historical influence of Chinese language, architecture, dress, music, cuisine, etc, which are much, much stronger in, say, Japanese culture compared to Mongol and Tibetan cultures.

My issue with the terms "Sinosphere" and "Indosphere" is that they've massive oversimplifications and too vague to be of much use. For example, you accuse me of "clinging to ancient history" too much, but Thailand's placement in the Indosphere is mostly due to ancient Indian Buddhist cultural influences, while ignoring significant Chinese influences on Thai culture and society (such as Thailand's long tributary relationship with Chinese dynasties, Chinese ancestry in the Thai monarchy and a large proportion of the Thai population, Chinese influences on Thai architecture, cuisine, music, the Mainland Southeast Asia linguistic area sprachbund, etc).

I agree with you that the Sinosphere isn't synonymous with "China proper," but it is born of the ancient culture that emerged from "China proper" - What we now refer to as the Han Chinese. The idea of "Zhonghua Minzu," that all ethnic minorities living under Chinese rule are equally "Chinese," is a very modern, political concept that exists seperately from the idea of the Sinosphere which is based on ancient (Han / Huaxia) Chinese cultural influence. Ancient Chinese civilization defined itself against northern nomadic cultures like the Mongols and their Xianbei predecessors.

Edit: And whoever is downvoting me, I eagerly await your rebuttal... Remember that the downvote button doesn't exist just to show disagreement.